By way of a specific example in the background art, the structure of an optical encoder as an electronic device will be described. The “optical encoder” is an electronic device used as a rotation detector for a servo motor used in the field of industrial equipment, in particular in the factory automation field. The optical encoder is an electronic device which incorporates a printed circuit board and which requires high rigidity as well as compact and lightweight design.
FIG. 4 is a cross-sectional view of the general configuration of a known optical encoder in the related art. FIGS. 5 and 6 are cross-sectional views of the general configurations of other examples of known optical encoders in the related art.
In FIG. 4, the base part 101 of an optical encoder that requires rigidity is generally made of a metal, and often of aluminum die cast to enhance mass productivity. A rotary part 102 is a pattern disc coupled to a rotary body (not shown) such as a rotor in a motor. An insulating part 103 is a component made of resin and insulates the base part 101 from a printed circuit board 104. A printed circuit board 104 mounts a light-receiving element, a light-emitting element, or an electronic circuit for performing signal processing (described later). An insulating part fixing screw 105 is used to fix the insulating part 103 onto the base part 101. A detector 106 is mounted on the printed circuit board 104.
In the example of FIG. 4, the fixing screw 105 is required to fix the insulating part 103. Positioning of the base part 101 and the insulating part 103 is made via this fixing screw 105. Thus, accuracy of positioning could be reduced by a clearance between the outer diameter of the fixing screw 105 and the screw hole in the insulating part 103. Unless the relation between the detector 106 and the pattern disc as the rotary part 102 is ultimately in a proper state, the performance of detecting the position of a rotary body as a function of an encoder will be substantially influenced. The use of the fixing screw 105 leads to an increase in the number of components, mounting man-hours and product mass.
In FIG. 5, a base part 101, a rotary part 102, a printed circuit board 104, and a detector 106 are components having the same functions as those in FIG. 4 so that the corresponding description is omitted.
The optical encoder shown in FIG. 5 includes the base part 101 and the printed circuit board 104 and thus features high rigidity. Disadvantageously, the printed circuit board 104 is in direct contact with the base part 101 made of a metal so that no components or patterns can be arranged on a portion where the printed circuit board 104 is in contact with the base part 101. A hatched portion shown as an insulating part 104a in the printed circuit board is the portion where no components or patterns can be arranged. This reduces the area of a printed circuit board that can be used effectively. This problem is serious in an application of compact electronic devices.
In FIG. 6, a base part 101, a rotary part 102, an insulating part 103, a printed circuit board 104 and a detector 106 are components having the same functions as in FIG. 4 so that the corresponding description is omitted. An adhesive part 301 shown in FIG. 6 is intended to bond the base part 101 to the insulating part 103. The base part 101 and the insulating part 103 in FIG. 6 are bonded to each other at the adhesive part 301 by an adhesive thus possibly reducing the accuracy of positioning of the base part 101 and the insulating part 103. Unless the relation between the detector 106 and the pattern disc as the rotary part 102 is ultimately in a proper state, the performance of detecting the position of a rotary body as a function of an encoder will be substantially influenced.
The adhesive part 301 is interposed between a base part 1 and an insulating part 3. An adhesive having reduced rigidity could reduce the rigidity of the entire encoder. The adhesive part 301 could be degraded by environmental conditions such as vibration, temperature and impact, thus reducing product reliability. Other possible drawbacks include an increase in the working man-hours required for bonding, an increase in the cost by the use of a potent adhesive, and reduction in the reliability due to variations in the bonding force caused by different environmental conditions in the manufacturing process.
Concerning the technology of manufacturing an electronic device housing, it has been proposed to mold a resin member and an aluminum die cast member as an integrated body and provide heat shielding as well as facilitate construction (for example, refer to Patent Reference 1).    Patent Reference 1: JP-UM-A-63-188519